Method of electrode plating silver on magnesium



United States Patent Otfice 3,427,232 Patented Feb. 11, 1969 3,427,232 METHOD OF ELECTRODE PLATING SILVER N MAGNESIUM John W. Natwick, Palo Alto, Calif., assignor to the United States of America as represented by the Secretary of the Air Force No Drawing. Filed Mar. 13, 1967, Set. N 623,513 US. Cl. 204-32 3 Claims Int. Cl. C2311 7/08, /62, 5/26 ABSTRACT OF THE DISCLOSURE A process for electrodepositing an adherent coating of silver on the surface of a magnesium work piece in order to provide a low resistance current path for microwave energy transmission. The process comprises the steps of cleaning the magnesium work piece surface in a basic solution by generating hydrogen thereon, etching the surface in a dilute solution of carboxylic acids and then immersing the work piece into a plating solution containing silver ions and a combination of alkali metal phosphate salts to effect deposition of the silver.

This invention relates to a method of electroplating metals. More particularly, it concerns itself with an improved method for providing a magnesium or magnesium alloy work piece with an adherent continuous coating of silver. Previous attempts to produce an electroplate of silver on magnesium or magnesium alloys have not proved successful. For example, cyanide plating baths fail to produce a continuous adherent coating of silver. Although deposition is accomplished, the resultant silver plate is easily corroded and fails to adhere upon bending or working.

The use of acid or aqueous plating bath have also proved difiicult. Apparently, the difficulty resides in the fact that magnesium is relatively high in the electromotive series and tends to enter solution or form oxidic coatings while immersed in the plating bath. As a result, electroplated coatings of silver on magnesium work pieces cannot be obtained by means of a simple commercially useful process.

Accordingly, the primary object of this invention is to provide a new and improved process for electroplating magnesium and alloys of magnesium.

Another object of this invention is to provide a novel means for coating the surfaces of a magnesium work piece with silver.

Still another object of the invention is to provide a magnesium work piece with a continuous, adherent and uniform coating of silver.

Further objects and advantages of this invention will become apparent from the following detailed description.

The above objects are obtained in accordance with this invention by a process which consists of a series of steps wherein the surface of the magnesium work piece is first cleaned in a basic solution by generating hydrogen on its surface. The cleaned metal surface is then etched in a dilute solution of carboxylic acids. Following the etching step, the work piece is immersed in an aqueous plating solution consisting of silver ions complexed in higher phosphates and amino compounds so that a suitable concentration of metallic ions may be maintained in solution under controlled hydrogen ion concentrations.

It has been found that, under optimum temperature and pH conditions, high polymers of phosphoric acid react with the alkaline earth metals to provide an active surface receptive to the deposition of an adherent coating of silver. The work piece is made the cathode in an electrochemical circuit and a direct current is applied during the coating process so that initially, metal deposition is minimized due to polarization effects and, finally, deposition proceeds with increasing efficiency to provide for intimate contact between the two metals at the deposited metal interface.

In carrying out the process of this invention the work piece to be plated, which may be either cast, rolled or a wrought material, is immersed in a cleaning solution heated to a temperature of from 180 to 210 F. The cleaning solution consists of a combination of tri-sodium phosphate, sodium hydroxide, and sodium metasilicate in the following range of proportions.

1-3% by weight of Na PO .12H O, 6-12% by weight of Na SiO .5H O, and 15-17% by weight of NaOH.

The article to be plated is arranged in the cleaning solution such that a direct current of to 150 amperes per square foot of surface is applied thereby generating hydrogen gas at its surface. The current is applied intermittently in five second periods for a total cleaning time of about thirty seconds. The article is then removed from the cleaning solution and immersed in Water heated to about 70 F. The water is agitated vigorously to remove the strongly attracted caustic film. The work piece is then transferred immediately to an acid etching bath maintained at 70-90 F. consisting typically of 1-2% by volume of acetic acid, 5-10% by weight of oxalic acid, and 340% by volume of nitric acid. Agitation of the etching bath is essential to maintaining rapid solution of alloying ingredients which appear on the metal surface as colloidal particles. The article is pickled for about two minutes, rinsed in cold water, and then transferred immediately to the plating solution which is operated at l20-140 F. The plating solution is made up by dissolving gms. of Na P O gms. Of Ag HP O 50-175 gms. of Na P O 3545 gms. of Tetrasodium ethylene diamine tetra acetic acid and 3-5 gms. of NaF in suflicient pure water to make up to exactly one liter of solution. Direct current is applied to the work piece during the plating operation so as to maintain it cathodic to the solution at about 5-25 amperes per square foot. Current contact is made prior to immersion in the plating solution. The application of current is maintained initially at a high value so as to permit cathodic reduction of high valance compounds and dissolution of the matrix metal to provide a freshly etched metal surface for adhesive deposition of the silver ions of the plating solution. High current application prevents deposition until the surface is cleansed of impurities. This is detectable by an increase in hydrogen over-voltage, at which time the current flow is decreased to a value within the limits stated above.

The pH of the solution is maintained above 7.9 to prevent orthophosphate formation. An optimum pH value is 8.3, although values as high as 9.5 may be used successfully on magnesium. The pH may be adjusted with triethanolamine and/or sodium bicarbonate.

The composition of the cleaning bath, etching bath, and plating bath may be varied within the approximate limits set forth herein. The following are specific examples of illustrative solutions which may be employed with the process of this invention.

'Example 1.Cleaning solution for magnesium and magnesium-base alloys A relatively dilute aqueous solution containing 2 percent by weight of Na PO 12H O, 9% by weight of Na SiO 5H O and 16% by weight of sodiumhydroxide.

The magnesium work piece is placed in this solution and a direct current of amperes per square foot of article surface is applied at intermittent periods of five seconds for a total cleaning time of thirty seconds.

3 Example 2.-Etching solution for magnesium and magnesium-base alloys An aqueous solution containing 1.5% by volume of acetic acid, 8% by weight of oxalic acid and 6% by volume of nitric acid provides a suitable etching solution in which the article to be plated is immersed. The solution is maintained at a temperature of approximately 80 F. for a period of about two minutes. Rapid agitation of the article is maintained during the etching process.

Example 3.Silver plating solution for magnesium and magnesium base alloy This solution is made up by dissolving 130 gms. of Na P O gmS. Of Ag HP O grns. Of N34P207, 40 gms. of tetrasodium eythylene diamine tetra acetic acid and 4 gms. of NaF in sufi'icient pure water to make up one liter of solution. The pH is maintained at a value of about 8.3. The article to be coated is placed in the plating solution operated at about 130 F. A direct current is initially applied to the article so as to maintain it cathodic to the solution at about 35 amperes per square foot. The current is then reduced to about amperes per square foot. An excellent plate of silver metal is obtained on the surface of the magnesium Work piece.

The process of this invention has proven to be extremely valuable in providing a method whereby a low resistance current path for microwave energy transmission can be provided on the surfaces of a magnesium or magnesium alloy work piece. The resultant silver coating is uniform and adheres tenaciously to the work piece surface. Also, it is to be understood that while there have been given certain specific examples of the practice of this inventon, it is not intended that the invention be limited to the specific details of the materials, proportions or conditions herein specified. Accordingly, modifications of the invention will became apparent t those skilled in the 4 art and all such modifications as come within the scope of the appended claims are intended to be included herein.

What is claimed is:

1. A method for electrodepositing a continuous adherent coating of silver directly on the surface of a magnesium work piece which comprises the steps of subjecting the work piece to the action of a hydrogen generating cleaning solution operated at a temperature of about 180- 210 F., etching the work piece in i8. oxalic acid bath operated at a temperature of about 90 F., and electrolyzing the work piece as the cathode in an electroplating bath consisting essentially of an aqueous solution of silver ions made up by dissolving to 160 grams of Na P O 35 to 45 grams of Ag HP O 50-175 grams of Na P O 3545 grams of tetrasodium ethylene diamine tetra acetic acid, and 3 to 5 grams of NaF in one liter of water.

2. A method according to claim 1 in which the temperature of the electroplating bath is to F.

3. A method according to claim 2 in which the work piece is maintained cathodic to the electroplating bath by the application of a direct current of about 5 to 25 amperes per square foot.

References Cited Electrochemical Society, Preprint 81-6, Nickel Plating Magnesium Alloys p. 53.

The Monthly Review of American Electroplaters Soc., Mar. 1943 p. 276.

Plating, Dec. 1950, pp. 12651269.

HOWARD S. WILLIAMS, Primary Examiner. W. VAN SISE, Assistant Examiner.

U.S. Cl. X.R. 20429, 46, 141 

